1. Field of the Invention
The present invention relates to a method of molding a magnetic tape cassette which overcomes a defect in the appearance of the cassette which is produced when forming a cassette half by multi-color molding. The invention also relates to the thus-molded cassette.
2. Background
Generally, a magnetic tape cassette used, for example, with an audio unit, a video unit or the like, has a pair of hubs rotatably housed in a case body composed of upper and lower cassette halves, and a magnetic tape is wound on these hubs.
Generally, the upper and lower cassette halves are injection-molded of a plastics material. Namely, a molten material is injected via an injection gate, such as a direct gate and a submarine gate, into a cavity which is formed by two mold members and corresponds in shape to the cassette half, thereby molding the cassette half. Then, the material filled in the mold cavity is cooled and solidified to provide a cassette half mold product which is removed from the above mold members. Thereafter, in order to achieve a design effect, a paper sheet having a predetermined design is affixed to the outer surface of the cassette half, or a predetermined design is printed directly onto the outer surface of the cassette half.
In order for the condition of winding of the magnetic tape to be visually confirmed from the exterior, the upper and lower cassette halves are made entirely of a light-transmitting material, or as shown in FIG. 8, a transparent or translucent window portion 6 is provided on an opaque cassette half body 2a. In order to achieve the design effect of the case body 1, the window portion 6 is formed into a predetermined shape.
Conventionally, the window portion 6 is composed of a molded window member separate from the cassette half body 2a, and is bonded to the cassette half body 2a by an adhesive, ultrasonic welding or the like. In this case, however, since the steps of molding the window member and of adhesive bonding the window member to the cassette half body are required, the design is limited, and the productivity is poor. Therefore, recently, a production method using two-color molding has been increasingly used.
At present, the most commonly known method of producing the above cassette half by two-color molding is a core back method or a core cavie back method. The process of producing the above cassette by a core cavie back method will now be described with reference to FIGS. 9 and 10.
FIGS. 9 and 10 are cross-sectional views of a cassette half-producing mold taken along a line 9,10--9,10 of the audio compact cassette of FIG. 8. As shown in FIG. 9, the injection mold for the core cavie back method is composed of a fixed mold member 20, a movable mold member 22, a slide cavity 26 provided in a slide hole 24 in the fixed mold member 20, a slide core 30 provided in a slide hole 28 in the movable mold member 22, and a gate block 25 provided in the slide cavity 26. A mold cavity 32 for molding the above cassette half 3a, 3b is formed at the mating surface between the fixed mold member 20 and the movable mold member 22. The slide cavity 26 and the slide core 30 for molding the window portion 6 are projected into the mold cavity 32 and are abutted against each other.
A submarine gate 35, communicating with a runner 33 and a primary hot runner 50 formed in the fixed mold member 20 for injecting a primary opaque or transparent resin, communicates with the mold cavity 32. Additionally, a secondary hot runner 34, formed in the gate block 25 inside the slide cavity 26 of the fixed mold member 20 for injecting a secondary transparent resin, communicates with the mold cavity 32. In FIGS. 9 and 10, the secondary hot runner 34 is formed in the gate block 25.
For molding the cassette half body 2a, the fixed mold member 20 and the movable mold member 22 are mated together, and then the slide cavity 26 and the slide 30 are abutted against each other by a hydraulic drive mechanism (not shown) at a position generally central to the thickness of the mold cavity 32. In this condition, the primary resin is injected from the submarine gate 35. Then, as shown in FIG. 10, the slide cavity 26 and the slide core 30 are retracted to form a window cavity 36, and thereafter the secondary resin is injected from the secondary hot runner 34 into the window cavity 36 to form the window portion 6.
Therefore, the cassette half body 2a and the window portion 6 are welded together and integrally molded together by heat and injection pressure produced during the injection molding. Referring to FIG. 11, in order that the welded portion between the window portion 6 and the cassette half body 2a be able to withstand a force acting in a direction of the thickness, the slide cavity 26 and the slide core 30 are different in size from each other so as to form a step portion 7 on the cassette half body 2a, which step portion corresponds to a peripheral edge portion 8 of the window portion 6, thus providing a double-layer portion formed by the primary and secondary resins.
The problem with such a method of forming the cassette halves 3a, 3b having the window portion 6 formed by such two-color molding, is that a drag 9 is liable to develop particularly at the step portion 7, as shown in FIG. 11. Specifically, before the step portion 7 of the window cavity 36 is adequately cooled, the secondary resin is injected and tends to be distributed over the peripheral edge portion 8. Due to the temperature of this resin and the injection pressure, the step portion 7 accumulates drag and is deformed.
The drag 9 results in a defect in the appearance, such as a striped pattern, and can be viewed through the outer surface of the cassette half 3a, because the window portion 6 is transparent.